Manufacture of electronic assemblies often makes use of a technique called “step-soldering.” In step-soldering, solder alloys of decreasing soldering temperature are used to bond a die to a substrate, a substrate to a package, and a package to a housing or printed wiring board. When made in a step soldering process, solder bonds made in a prior operation do not reflow when subsequent operations are performed at step by step decreasing temperature decrements. The operation that is performed first, generally die to substrate bonding, uses a solder that has a soldering temperature that is higher than that of any solder used in a subsequent operation. Consequently, subsequent soldering operations do not disturb the previously soldered components.
In microelectronics and optoelectronics manufacturing, solder alloys are generally gold-based, like gold-tin and gold-germanium, to provide a high soldering temperature and to provide compatibility with gold and other precious-metal metallization used for electrical contact between a semiconductor die and substrate.
Solder pads are applied by depositing alternating layers of the desired final alloy constituents in separate layers, for example: gold, tin, gold, tin, gold, tin with eleven layers total not being unusual. The layered structure is heated, and, upon reaching a soldering temperature which is above the melting point of tin, the metals inter-diffuse to form the desired solder alloy. Note that a dwell time at a temperature below melting may also be used to achieve inter-diffusion of the layers. For example, inter-diffusion is essentially complete after one second at 210° C. In a step-solder process, each subsequent step occurs at a soldering temperature lower than the previous soldering step so as not to remelt previous solder connections. To be robust in a manufacturing environment, there should be a large difference in the soldering temperature between one solder step and the next.
From a practical perspective, step-soldering is limited to about three or four steps between the first solder used and the last. The last solder used is typically a tin-lead solder having a low soldering temperature. For this reason, conventional step soldering has difficulty accommodating the mounting of multiple dies on a substrate in multistep mounting operations. It would be desirable to be able to precision place a die, like a gallium arsenide laser, on a substrate such as silicon, to solder it into position, to move the substrate, and then to repeat the operation to precision mount another die, like a photodetector chip. Thus, a need exists for a configuration of solder pads which facilitates the mounting of multiple components in a multi-step process.